An aircraft is generally associated with a flight envelope that describes its safe performance limits. Flight envelope protection (FEP) design can deter or prevent the pilot from making control inputs that would put the aircraft outside of these predefined, safe performance limits. In fly-by-wire controlled aircraft, various forms of envelope protection have been implemented in both military and commercial aircraft. The FEP systems in fly-by-wire controlled aircraft are unsuited for mechanically controlled aircraft because the mechanical link between the control yoke and control surfaces of the aircraft does not allow for independent movement of the yoke or surface. In a mechanical flight control system, it is desirable to provide the pilot with a resistance force that can be felt via the pilot's hand on one or more control mechanisms, using a torque controlled servo when the pilot is manually flying the aircraft outside the flight envelope. This torque controlled servo should provide little or no resistance force that may be felt by the pilot when the pilot is intentionally performing the recovery maneuver and bringing the aircraft back inside the flight envelope.
One example of a servo is a flight control actuation servo, such as that used by a typical autopilot system on an aircraft. Flight control actuation servos used by most autopilots are typically designed with very high mechanical advantage in order to supply sufficient torque on the control surfaces while using the smallest and lightest direct current (DC) motor possible within the servo. In these designs, when the autopilot is engaged, the autopilot servo alone will drive the control surfaces, and is not back-drivable by the pilot. In order to contribute to manipulation of the flight control surfaces, the pilot must either disengage the autopilot or forcibly overpower it. In older actuation servos, overpowering the servo caused a slip-clutch to slip or a shear pin to break. In newer actuation servos, overpowering the servo results in electronic clutch disengagement when the sensed motor current monitored by a current loop within the servo exceeds a predefined threshold. Under normal flight operations, the aircraft flight control actuation servo, after engaged, is operated exclusively by the autopilot, with no commanding force being provided by the pilot through the mechanical linkage. Here, the pilot is “not in the loop,” and the pilot is not operating the aircraft flight control system. A flight control actuation servo is generally a position controlled device with aircraft control surface displacement (position) being the feedback signal or equivalently servo pushrod travel displacement as feedback. Thus, the position loop control is the basic operating mode for most of the flight control actuation servos when operating on the aircraft primary control surfaces, e.g., aileron, elevator, and rudder. In this design, the sensed motor current is utilized to produce an additional torque controlled operating mode for the servo.
For purposes of this application, the actuation servo is used as the torque controlled device while the pilot is providing control torque to the aircraft control surfaces, thus the “pilot in the loop” condition. In this case, the surface control is not an either/or proposition, (either the pilot or autopilot, but not both) but rather, both the servo's applied torques and the pilot's manually applied torque contribute to the deflection of the flight control surfaces. There are two potential interactions between the pilot and an actuation servo under this pilot in the loop condition: (1) the pilot's torque resists the servo torque; or (2) the pilot's torque aids the servo torque. Furthermore, when the pilot's torque aids the servo torque under condition (2) and when the actuation servo's motion starts lagging behind the pilot induced surface motion, the servo generates a significant resistance torque induced by a back electromotive force (EMF). It is known that the motor torque produced by back EMF is in the opposite direction with the turning direction of the servo and that the magnitude of back EMF increases with rotational speed. This back EMF force feels like a “kick back” on a control mechanism in the pilot's hands, creating difficulty for a pilot to perform a recovery maneuver.
Accordingly, it is desirable to provide a system for compensating this pilot aiding load and mitigating this back EMF force, which may be felt by the pilot's hands, when the pilot is intentionally performing a recovery maneuver to bring the aircraft within the constraints of the flight envelope. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.